Many prosthetic medical devices are implanted into load-bearing joints such as knees, hips, etc. As such, these prosthetic devices must be very strong and possess a high degree of wear resistance. The prosthetic medical device industry has utilized various methods and compositions employing metals and polymers and combinations thereof to fabricate prosthetic devices. Prosthetic medical devices manufacturers constantly work toward developing better products by improving their physical properties. Improved wear resistance, for example, is a desirable quality to impart to a prosthetic medical device. Improving wear resistance without losing strength or causing oxidative degradation is a difficult balance to obtain.
Various methods of manufacturing compositions of polymeric materials have been devised with the objectives of reducing wear rate and improving the oxidation resistance of the polymeric materials used to fabricate prosthetic medical devices. U.S. Pat. Nos. 6,017,975, 5,879,400, 5,414,049 and 5,728,510 are referenced herein to illustrate the common methods and compositions used to fabricate polymeric prosthetic devices presently employed in the field.
One common practice within the prosthetic medical device industry is to use cross-linked polymers and resins to form the medical device. “Cross-linked” polymers are defined as polymeric materials which have been subjected to chemical or radiation-initiated activation resulting in dendritic bond formation between and amongst individual polymeric chains yielding new intermolecular and intramolecular networks. These cross-linked networks within the polymer provide chemical and physical properties which are usually different from the virgin polymer. Such properties include increased wear and creep resistance, durability, etc. Indiscriminate or uncontrolled cross-linking of the polymeric material comprising the prosthetic device may result in improved wear resistance, but strength and other desirable properties may be sacrificed.
Another difficulty conventionally encountered in the manufacturing process of polymeric components of ball and socket or bearing-type prosthetic medical devices, such as hips, knees, and other load-bearing joints, is that they cannot be formed easily by inexpensive injection molding techniques. Instead, these particular types of prostheses must first be formed into a stock bar or rod, by extrusion for example, after which further machining is necessary to form the finished article. Injection molding, on-the-other-hand, allows for the final article to be formed in virtually one step.
Therefore, a need exists within the prosthetic medical device industry to fabricate an improved polymeric prosthetic device possessing sufficient strength to withstand the stress and pressure imposed on it, yet resist wear. There also exists a need to fabricate the devices inexpensively by injection molding. The present invention provides compositions, as well as methods of improving the wear resistance of prosthetic medical devices, by selectively cross-linking a polymeric resin using a controlled cross-linking process providing improved strength and wear resistance.
The present invention also provides compositions and methods of injection molding and selectively cross-linking prosthetic medical devices thus rendering an inexpensive, and more facile prosthetic medical device fabrication process.